Nothing is jumping out at me from first look at your xml.
I do notice you have the kernel set to 35KHz when you need well under 25KHz, so might be worth setting to 25 and restarting Mach.
You could also try reducing the look ahead to see if that helps.
Might also be worth checking the integrity of the Dir wiring on your Z axis or swapping over drives from another axis to see if the fault stays with the Z or moves with the drive.
Hood